ppmtogif: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/ppmtogif - from package: netpbm-progs

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/ppmtogif - from package: netpbm

You may install the required package with command apt-get install {package}

Error in other languages:
ppmtogif: bevel nie gevind nie
ppmtogif: comandă negăsită
ppmtogif: comando não encontrado
ppmtogif: commande introuvable
ppmtogif: command not found
ppmtogif: käsku ei ole
ppmtogif: không tìm thấy lệnh
ppmtogif: komanda nerasta
ppmtogif: Kommando nicht gefunden.
ppmtogif: kommandot finns inte
ppmtogif: komut yok
ppmtogif: nie znaleziono polecenia
ppmtogif: níor aimsíodh an t-ordú
ppmtogif: no se encontró la orden
ppmtogif: no s'ha trobat l'ordre
ppmtogif: opdracht niet gevonden
ppmtogif: parancs nem található
ppmtogif: perintah tidak ditemukan
ppmtogif: príkaz nenájdený
ppmtogif: příkaz nenalezen
ppmtogif: команда не найдена
ppmtogif: командата не е открита
ppmtogif: 命令找不到
ppmtogif: コマンドが見つかりません